This invention relates to the production of lower alcohols. The production of lower alcohols such as butyl or propyl alcohols are among the most commercially significant operations undertaken in the chemical industry. The principle means by which these alcohols are manufactured is through the hydration of olefins. Hydration may either be direct, wherein an olefin is catalytically reacted with water, or it may be indirect. In indirect hydration processes olefinic starting materials are passed into a mixture of sulfuric acid and water. Intermediate carbonium ions are formed at the site of the double bond in the starting material. These carbonium ions then react with sulfate ions to form alkyl hydrogen sulfate and dialkyl sulfates which are then reacted with water to give the alcohols.
In industrial applications, many factors complicate these otherwise straightforward indirect hydration reactions. For example, numerous intermediate and products of side reactions such as etheric derivatives of the reactants accompany the production of the desirable alcohol products. Where production accompanies petroleum based refinery operations, it is not uncommon to have feeds mixed with small amounts of other substances. For example, in the production of secondary butanol from butylene it is not uncommon to have small amounts of isobutylene, isopentene, and other impurities mixed with the desired n-butylene feeds. This can result in copolymerization of these species along with the production of the desired products. Octylene, hexadecene, dodecene, and high homologues have been identified as several oligomeric and polymeric substances that readily form in this process. These polymeric substances result from exhaustive dehydration, affected by concentrated H.sub.2 SO.sub.4, of the various oligomeric species of alcohols and ethers, with very small contribution from sulfonated and sulfated species. Elemental analysis yields a typical composition of C.sub.8 H.sub.12 O.sub.1 S.sub.0.1. These substances and others contribute to the presence of undesirable carbonaceous particulate matter. Other impurities are also generated in these processes such as ethers, diethers, sulfated versions of polymeric, oligomeric, and other byproducts. Even beyond creating a yield loss of desired product, these polymeric impurities create significant problems in processing operations. In one method of creating high purity product, phase separators follow reaction vessels so that fat acid intermediates and undesirable or unreacted hydrocarbons can be separated. Thereafter, the hydrocarbons are either recycled or used in other processes. The presence of polymers can show up in separators as a rag layer between the two phases. Tight emulsions can form so that separation becomes more complex and takes more time thereby diminishing the efficiency of the process.
Additional problems include excess hydrocarbon entrainment and acid carryover in steps required to separate or finish the desired product. In the production of secondary butanol, for example, butylene is frequently entrained to fat acids after olefin hydration. Separation of the desired product typically requires at least one stripping step. The presence of polymeric impurities can cause phase separation problems resulting in excess butylene entrainment. This causes foaming in the strippers which also reduces process efficiency.
Further still, the presence of solids in steps involving liquid flow can cause innumerable mechanical problems such as fouling transmission lines, clogging valves, and clogging filters. In spite of the problems created by the presence of polymers in alcohol production, some of the polymers have utility. For example, some C.sub.4 and C.sub.5 based oligomers can be added to good effect in gasoline blends. Thus, removing such impurities from alcohol production processes could not only allow the production of alcohols to proceed more efficiently but could also provide a mechanism for capturing useful byproduct for subsequent use elsewhere such as in gasoline blending.
Removal of polymeric and carbonaceous materials from butyl alcohol production would greatly enhance the efficiency and economics of production. Unfortunately, many conventional methods have proven unacceptable to accomplish these objectives. A liquid phase water/caustic/water wash sequence of phase separated crude secondary butyl alcohol, for example, is known in the art. It is useful for the removal of some unwanted substances such as unreacted C.sub.4 entrained to acid. However, this process does little to remove solid polymeric species.
One approach taken to address these problems is to discard or continuously draw off H.sub.2 SO.sub.4 so that entrained materials are removed with it. U.S. Pat. Nos. 3,227,774, 3,462,512 and 3,691,252 to Goldsby disclose methods employing this technique. In these methods, polymeric oils are removed by an acid extraction in isobutane. Unfortunately, this method does not apply to the removal of particulate matter and requires careful control of reaction conditions to avoid yet further polymerization. Further, a considerable amount of acid is "spent" and must be disposed. Continuously feeding H.sub.2 SO.sub.4 to compensate for that which is drawn off can become rather expensive.
U.S. Pat. No. 3,733,368 to Dodd et al. discloses a method for removing polymeric impurities in the catalytic hydration of ethylene to ethanol. This involves exposing an ethylene recycle stream to a series of two separate treatments with different scrubbing reagents. Only lighter polymeric impurities are effected. U.S. Pat. No. 4,762,616 to Litzen et al. provides a method for removing polymeric impurities from isopropyl alcohol. This occurs only during the finishing phase of alcohol production. It is a purification method after the bulk of the process reactions have already occurred. Both of these patents are broadly representative of prior art methods for impurity removal; the focus is on cleaning the products or their precursors rather than on purifying the acid during the hydration process.
Rindtorff et al., Canadian Pat. 703,171, prevent introduction of polymeric material into recycle streams of ethylene in a similar process. In this process, the entire hydration reactor product is washed first with ethanol and then with water. The use of other solvents such as benzene and toluene is also disclosed. Still, the product of this wash step joins the aqueous ethanol stream to be purified downstream. Thus, the impurities therein dissolved can form incrustations resulting in problems in subsequent steps similar to those noted above.
Throughout much of the prior art, removal of impurities from alcohol production is facilitated by maintaining the substances to be treated in the gaseous state. The Rindtorff process above and U.S. Pat. Nos. 3,953,533 and 4,351,970 to Sommer et al, share this characteristic. Each is designed for the olefinic hydration at high temperatures and is inapplicable to the removal of polymers in process steps that might otherwise be conducted in the liquid state.
Processes for producing lower alcohols by the hydration of olefins could be made much more efficient and economically beneficial if polymeric impurities could be removed during the course of the reactions. This is particularly true where such processes occur in the liquid state. Safety concerns and other factors which make operating with vaporized reactants unattractive, increasingly make such liquid state operations desirable. Eliminating fouling agents from entering the mechanical components of these processes and reducing detracting phase changes such as foaming is greatly needed.